Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier modulation scheme in which the wide transmission spectrum is divided into narrower bands and data is transmitted in parallel on these narrow bands. Therefore, symbol period is increased by the number of sub-carriers, decreasing the effect of inter-symbol interference (ISI). The remaining ISI effect is eliminated by cyclically extending the signal. OFDM provides effective solution to high data-rate transmission by its robustness against multi-path fading. Parallel with the possible data rates, the transmission bandwidth of OFDM systems is also large. UWB-OFDM and IEEE 802.16 based wireless metropolitan area network (WMAN) systems are examples of OFDM systems with large bandwidths. Because of these large bandwidths, noise can not be assumed to be white with flat spectrum across subcarriers.
The signal-to-noise ratio (SNR) is broadly defined as the ratio of the desired signal power to the noise power and has been accepted as a standard measure of signal quality. Adaptive system design requires the estimate of SNR in order to modify the transmission parameters to make efficient use of system resources. Poor channel conditions, reflected by low SNR values, require that the transmitter modify transmission parameters such as coding rate, modulation mode etc. to compensate for the channel and to satisfy certain application dependent constraints such as constant bit error rate (BER) and throughput. Dynamic system parameter adaptation requires a real-time noise power estimator for continuous channel quality monitoring and corresponding compensation in order to maximize resource utilization. SNR knowledge also provides information about the channel quality which can be used by handoff algorithms, power control, channel estimation through interpolation, and optimal soft information generation for high performance decoding algorithms.
The SNR can be estimated using regularly transmitted training sequences, pilot data or data symbols (blind estimation.). In conventional SNR estimation techniques, the noise is usually assumed to be white and an SNR value is calculated for all subcarriers. In addition, in the prior art it is known to remove this assumption by calculating SNR values for each subcarrier. However, the correlation of the noise variance across subcarriers is not used since noise variance is calculated for each subcarrier separately.
White noise is rarely the case in practical wireless communication systems where the noise is dominated by interferences which are often colored in nature. This is more pronounced in OFDM systems where the bandwidth is large and the noise power is not the same over all the sub-carriers. Color of the noise is defined as the variation of its power spectral density in frequency domain. This variation of spectral content affects certain sub-carriers more than the others. Therefore, an averaged noise estimate is not the optimal technique to use.